The invention relates to a method for reducing the thermal load of an automatic transmission for a motor vehicle in an emergency operating mode.
The utilization of the intelligence in electronics finds its expression in motor vehicles having modern automatic transmissions in the use of an electronic transmission control (EGS), which to a great extent, meets the known criteria to be relevant for the customers, such as driving comfort and drivability, and also relevant to security. For the selection of gear, according to situations, the electronic transmission control steadily communicates with other control units and computers of different aggregates via a CAN (controller area network) data bus.
For the case of a defect of the electronic transmission control, emergency operating programs have often been provided which are designed so that in an emergency operating mode a vehicle can be operated in the widest possible driving range. Depending on the emergency running, the cooling of the transmission is problematic in such an emergency operating mode, since the heat accumulation in the emergency operating mode of the transmission must be eliminated via the cooling system of the vehicle and, in case of complete failure of the electronic transmission control, the transmission has no positive way of its own to act upon the thermal load.
This particularly occurs in CVT (Continuously Variable Transmission) automatic transmissions which have a variator for continuous adjustment of a reduction ratio between a shortest possible ratio (LOW) and a longest possible ratio (overdrive, OD).
Such a CVT transmission with a first cone pulley pair upon an input shaft as primary pulley set and with a second cone pulley pair upon an output shaft as secondary pulley set has been described in xe2x80x9cATZ Automobiltechnische Zeitschriftxe2x80x9d 96 (1994). Each cone pulley pair consists of a first pulley axially stationary and a second pulley axially movable which are designated as a primary pulley and a secondary pulley whereby they belong to the primary or secondary pulley sets, respectively. Between the cone pulley pairs rotates a torque-transmitting element, for example, a linked band, wound around the cone pulley pairs. To adjust the primary pulley and the secondary pulley, they are loaded with a pressure medium from a pressure source, the pressure being applied, respectively, to the adjusting chambers of the primary pulley and secondary pulley with the aid of an electronic transmission control and an electrohydraulic pressure-supply device.
DE 44 36 506 has disclosed a device for control of a CVT transmission in which an electronic control unit determines, via electromagnetic actuators and hydraulic valves, the pressure level of the adjusting chambers of primary pulley and secondary pulley which are not dynamically balanced.
For a failure of the electronic control unit, an emergency operating device is provided which has two pressure-regulating valves, two pressure-reducing valves and at least one emergency valve, wherein a primary valve and a secondary valve adjust a constant static pressure ratio or power ratio between primary pulley and secondary pulley in constant static secondary pressure level.
By the amount that the static power ratio and the dynamic forces on the pulleys is not dynamically pressure balanced, the magnitude of the ratio change of the CVT transmission from a normal operation to the emergency operating mode can be established. The emergency operating mode is at the same time described as a state of the CVT transmission in which a rotational speed regulation and a contact regulation of the secondary pulley are interrupted.
In this known device, the ratio of the CVT transmission varies according to the generated torque of an internal combustion engine provided as input unit, wherein the variable ratio change in the emergency operating mode makes a reliable start on the hill and improved driveability at high top speed possible.
As input signals abut on an electronic control unit, e.g. the signal of a load position of the input unit like the throttle valve position in case of an internal combustion engine, the rotational speed of the transmission input shaft, the rotational speed of the output shaft, or the temperature of the pressure medium. From the input signal, an operating point is determined and the appertaining rotational speed of the transmission input shaft or the ratio of the CVT transmission adjusted. When an error occurs, the CVT transmission changes over to the emergency operating mode in which no rotational speed regulation and no contact pressure regulation of the secondary pulley take place any more.
Even though this known solution offers a wide possible adjustment range of the variator, it is problematic that the contact pressure for the corresponding pulleys must be designed according to the extreme points of the driveability range. One of the points is the guaranty of the maximum starting power in the lowest gear, i.e. in or near the LOW ratio. For the variator capacity of transmission, a higher contact pressure on the secondary pulley is required at high ratio and high input torque. As result of the constant static pressure ratio between primary pulley and secondary pulley, the static force adjusted on the secondary pulley, whose magnitude depends on the maximum input torque to be transmitted, is constant in the whole torque range.
In wide driving ranges, the consequence of this is an undesired overpressure, which disadvantageously leads to a high operating temperature in the variator with increased loss of effectiveness and wear of the variator system.
To reduce an inadmissible thermal load of the input engine of a vehicle, in WO 93/13340 has been disclosed a diagnosis module designed as self-sufficient control module detects erroneous conditions of the drive train via defined limiting value observations of sensor signals or input signals delivered by other control units of the drive train. Depending on faulty gravity, the diagnosis module more or less limits the maximum admissible vehicle speed. Combined with a transmission control, WO 93/13340 proposes to limit the vehicle speed by the gear selection of the transmission, the relevant gear of the transmission control being preset by the diagnosis module according to the maximum admissible vehicle speed, which presupposes a fully operable transmission control.
The problem on which this invention is based is to make a method available for reducing the thermal load of an automatic transmission for a motor vehicle in an emergency operating mode in which method the thermal and mechanical damages on the automatic transmission are prevented due to heat action during the emergency operating mode.
With the aid of the inventive method, a maximum vehicle speed v_max_not or maximum engine rotational speed n_mot_max_not is preset by the digital motor electronic system in the emergency operating mode, it is possible to prevent, by a simple limitation of the operating range, a thermal overstress and a heat-determined wear of components of the automatic transmission, the same as a thermal aging of the transmission oil.
In the inventive method, the necessary protective functions for the transmission are perceived by the engine electronic system whereby a more effective overheat protection is still ensured when the electronic transmission control has entirely failed, has been electronically disconnected, or has been electrically separated from the vehicle circuit electronics.
The limitation of the vehicle speed or the limitation of the engine rotational speed, which likewise entails a limitation of the highest speed of the vehicle, by a program module in the engine electronics system represents a simple, but effective step, both in regard to an electrohydraulically controlled stepped automatic transmission of conventional type and also in regard to a CVT transmission.
The use of the inventive method is of special advantage in CVT transmissions, since with only a slight limitation of the operating condition is prevented here; the entrance of heat in the variator due to overcontact with the attendant consequences of wear.
In addition by omitting extreme angular points of the driveability in the emergency operating mode, the dimensions of the transmission oil cooler can be kept small.